Image display apparatus and image display method

ABSTRACT

When displaying moving images by using liquid crystal panels and others, a problem exists in that tail streaks occur, and image degradation is resultantly caused. To solve this problem, an echo suppression circuit divides a video signal into a pair composed of two successive frames for comparison between these two frames in signal level for the purpose of suppressing an echo phenomenon occurring when a liquid crystal panel displays video signals including moving images. If these two frames are not the same in signal level, the signal is accordingly corrected so as to equalize the signal level of these frames. Based on the video signals and others corrected by the echo suppression circuit, a controller operates the liquid crystal panel with AC drive through a source driver and a gate driver. As such, the drive voltage used to operate the liquid crystal panel with AC drive is adjusted so as to be balanced between positive and negative. In this manner, the tail streaks are prevented when displaying moving images.

TECHNICAL FIELD

The present invention relates to image display apparatuses and, morespecifically, to image display apparatuses for displaying images basedon input video signals. The present invention especially relates toimage display apparatuses that are preferably applicable when displayingmoving images on liquid crystal display apparatuses.

BACKGROUND ART

FIG. 31 is a block diagram showing the structure of a conventionalliquid crystal display apparatus. In FIG. 31, the conventional liquidcrystal display apparatus includes a controller 910, a source driver911, a gate driver 912, and an IPS-type liquid crystal panel 913. Thecontroller 910 performs, as a main role, timing control of the sourcedriver 911 and the gate driver 912, and alternating-current (AC) drivecontrol of the IPS-type liquid crystal panel 913.

The AC drive of the liquid crystal panel is described below. In theliquid crystal panel, a liquid crystal material is used to seal betweentwo electrodes on a pixel basis so as to change the voltage to beapplied between these electrodes. Thereby, liquid crystal molecules arealigned differently, the optical property is accordingly changed, andimage display is thus performed. Generally, in a TN (TwistedNematic)-type liquid crystal panel, if a direct-current (DC) is appliedthereto as a driving voltage, ions in the liquid crystal material movecloser to the electrodes, and as a result, a phenomenon calledsticking-image occurs with some display images being stayed. This is thereason why the liquid crystal panel often operates with an AC drive, andis generally driven in AC waveforms in which the polarity alternates insynchronization with a vertical synchronizing signal of the videosignal. FIG. 32 is a diagram showing a drive voltage waveform of, undersuch a conventional AC drive method, a test pattern of still imagesshowing gray, white, and gray in the horizontal direction. The lateralaxis is a space axis, that is, the lateral axis indicates pixelpositions in the horizontal direction. Because of operation with an ACdrive, the polarity of the drive waveform alternates on a frame basis,that is, between an odd-numbered frame and an even-numbered frame. Here,as shown in FIG. 32, the polarity of the drive waveform also alternatesbetween any horizontally adjacent pixels, which is called dot reverse,or column reverse, and is one popular method which is used to reduceflicker often occurring at the time of the AC drive.

On the other hand, as a technique for improving the viewing angleperformance of liquid crystal displays, liquid crystal panels of anin-plane switching type (hereinafter, referred to as IPS (in-PlaneSwitching) type) have been recently developed. FIG. 33 is a set ofdiagrams showing the structure of such an IPS-type liquid crystal panel.Specifically, FIG. 33( a) is a diagram viewed from a directionperpendicular to its display surface, while FIG. 33( b) shows a sectionview. As shown in FIG. 33, in the IPS-type liquid crystal panel, twoelectrodes for driving a liquid crystal, i.e., a common electrode 921and a drain electrode 922, which is connected to a source line 920through a pixel transistor 923, are in such a comblike structure asoccluding each other on the same surface of a glass substrate 924. Inthe IPS-type liquid crystal panel, switching takes place with respect tothe liquid crystal of a liquid crystal layer 927 by a horizontalelectric field generated between these two electrodes 921 and 922,thereby advantageously realizing the property of quite a wide viewingangle. Here, because the liquid crystal is rather slow in responsespeed, the IPS-type liquid crystal panels are now mainly used as displaypanels for still images exemplified by monitors of personal computers.With the improvement of the IPS-type liquid panels and their relatingtechnologies, the IPS-type liquid crystal panels are becoming applicableto displaying moving images such as television signals.

The issue here is, if a liquid crystal display apparatus including aconventional IPS-type liquid crystal panel displays moving images suchas television signals, a problem surely arises due to liquid crystalbeing slow in response speed. Furthermore, the inventors have foundanother problem through their study in that movement of displayingobjects, patterns, and others in the moving images result in tailstreaks, which causes image degradation of the region to which thoseobjects, patterns and others moved.

With reference to FIGS. 33 to 37, described in detail below is amechanism why such a new problem of image degradation occurs if movingimages are displayed on the IPS-type liquid crystal panel in theconventional liquid crystal display apparatus.

Initially, the electrode structure of the IPS-type liquid crystal panelis described first in comparison with that of the general TN-type liquidcrystal panel.

Generally, in the TN-type liquid crystal panel, a planar transparentelectrode (ITO) is provided for each glass substrate placed so as toface each other. With such a structure, the planar ITO works as astopper when an insulator film over the ITO is removed in themanufacturing process, whereby etching can be done with no overetchingbeing caused. In the IPS-type liquid crystal panel, on the other hand,as shown in FIG. 33, pixel electrodes are provided in a comblikestructure (e.g., Al, Cr), that is, the common electrode 921 and thedrain electrode 922 are placed so as to occlude each other on the samesurface of the glass substrate. With such a structure, when insulatorfilms over those pixel electrodes, that is, a gate insulator film 925and a protection insulator film 926, are removed in the manufacturingprocess, electrodes such as the common electrode 921 and the drainelectrode 922 work as a stopper for their own part, but there is nothingworking as a stopper for a part between the common electrode 921 and thedrain electrode 922. Thus, without correct control over the etchingspeed, there is a possibility for overetching. This is the reason whythe insulator films over the pixel electrodes are often not removed inthe IPS-type liquid crystal panel, that is, the pixel electrodes remaincovered by the insulator films. This is one cause of the tail streaksmentioned above.

FIG. 34 is a set of diagrams showing drive voltage waveforms of theconventional liquid crystal display apparatus including the IPS-typeliquid crystal panel in the case where a test pattern showing white,gray, and white is moved by two pixels rightward on a frame basis. Inthese diagrams each corresponding to a frame, the lateral axes indicatepixel positions in the horizontal direction (space axes), the verticalaxes indicate the drive voltage, and the frames are arrangedlongitudinally in order (discrete time). As already described byreferring to FIG. 32, with an AC drive, the polarity alternates on aframe basis in the drive voltage waveform, and further, with a columnreverse, the polarity alternates on a pixel basis in the horizontaldirection in the drive voltage waveform.

Here, in FIG. 34, focusing on a pixel A shown therein, a time-baserepresentation of any change observed in the drive voltage will lead toa diagram shown in FIG. 35( a). As indicated by the thick line in FIG.35( a), DC components (low frequency components) of the voltage appliedto the electrodes become out of balance when the test pattern passesthrough. In other words, at the time when the test pattern passesthrough, the DC voltage is applied to the electrodes of the IPS-typeliquid crystal panel.

As described in the foregoing, the electrodes of the IPS-type liquidcrystal panel are each covered by an insulator film (SiNx), andtherefore with a DC voltage being applied to the electrodes as such,polarization occurs in the insulator films. FIG. 36 is a model diagramshowing how polarization occurs as a result of DC voltage application,(−) to the common electrode 921, and (+) to the drain electrode 922. Asshown in FIG. 36, DC voltage application to the IPS-type liquid crystalpanel causes ions in its liquid crystal layer to move, and due to aresultant uneven distribution of ions, polarization occurs both in theliquid crystal layer and the insulator films covering the electrodes. Asa result of such polarization, electric field components are generatedso that the electric field applied to the liquid crystal layer isthereby cancelled out. Moreover, the electric field components generatedas such keep affecting the electric field applied to the liquid crystalfor the duration until the polarization level is lowered.

FIG. 35( b) is a diagram showing an electric field applied to the liquidcrystal of the focusing pixel A. As indicated by the thick line in FIG.35( b), due to polarization resulting from an electrode voltage addedwith DC components, such an electric field component as canceling outthe DC components affects an electric field to be applied to the liquidcrystal during a pattern display period and thereafter. Here, focusingon the electric field especially after the pattern has passed through,in frames after the pattern has passed through, the voltage to beoriginally applied to the liquid crystal of the pixel A is the oneshowing no change in absolute value as shown in FIG. 35( a). However, asshown in FIG. 35( b), actually applied thereto is such a voltage asincreasing and decreasing in absolute value on a frame basis. As aresult, AC drive becomes out of balance between positive and negative,causing flicker. As described above, under AC drive, the polarityalternates in synchronization with the vertical synchronizing signal.Accordingly, such flicker occurs in half of the frequency components ofthe vertical synchronizing signal.

Such flicker increases in proportion to the size of the DC component andthe time when the DC component was applied. As an example, by firstdisplaying white for a positive frame and black for a negative framesequentially for two seconds each, and then displaying gray, a flickerresultantly occurs which is visible even to the naked eye. Also, even ifthe flicker that has occurred is in such a level as not being visible tothe naked eye when the line of sight is fixed, the flicker may becomevisible once the line of sight is changed. This is explainable by thehuman eyes as being a sensory organ sensitive to the amount of spatialand temporal changes. When the line of sight is fixed, only the amountof temporal change in brightness becomes a sensory stimulation, but whenthe line of sight is changed, in addition to the amount of temporalchange in brightness, the amount of spatial change in brightness alsobecomes the sensory stimulation. For example, as shown in FIG. 37, in adisplay screen 914 of the IPS-type liquid crystal panel, if an exemplarytest pattern of a white BOX 915 is moved leftward within a graybackground 916, the human eyes follow this movement. Sincesynchronization is established between the movement of the test patternand flicker, as indicated by the arrows in FIG. 38, the line of sighthas a directional property in the temporal and spatial direction, and asa result, flicker occurs as if a pattern of streaks is moving. As aresult, a tail echo 917 such as the one shown in FIG. 37 is perceived.As such, unlike general afterglow, the tail echo 917 appears as apattern of streaks, causing considerable image degradation of the movingimages.

Here, as described in the foregoing, one cause of the tail echo 917 isan uneven distribution of ions (liquid crystal polarization) as a resultof DC voltage application. This polarization occurs as a result ofimpurity ions in the liquid crystal panel moving in response to theelectric field. Accordingly, the polarization level is increased as thedensity of such impurity ions is increased in the liquid crystal panel.

Conventionally, in order to increase the response speed of a liquidcrystal material used for the IPS system, its viscosity has been on adownward path. Further, in order to lower the drive voltage, Δ∈(anisotropic dielectric constant) has been on an upward path. Throughsuch a development, the liquid crystal material for the IPS systemgenerally includes a CN liquid crystal, or the liquid crystal materialto be used therefor is high in ∈ (permittivity). However, with such aliquid crystal material including a CN liquid crystal or being high in∈, impurity ions are to be easily captured in the liquid crystal. As aresult, as already described, polarization occurs easily so that anelectrical charge on the resultant interface is increased.

Moreover, for the purpose of reducing a streaking-image, for example,the liquid crystal panel may be filled with liquid crystal of a lowresistance or provided with an orientation film, the liquid crystalpanel may be irradiated with a UV ray, or the liquid crystal therein maybe mixed with any additive. If these are the cases, however, the iondensity in the liquid crystal is resultantly increased so that theabove-described echo phenomenon occurs more apparently, considerablydegrading the quality of the moving images.

Therefore, an object of the present invention is to provide a liquidcrystal display apparatus and method in which no echo phenomenon occurseven if moving images are displayed by using a liquid crystal panel.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention has the followingaspects.

A first aspect of the present invention is directed to an image displayapparatus for displaying an image based on an input video signal. Theimage display apparatus comprises: a display device for outputting imagelights according to a voltage to be applied;

drive means (unit) for driving the display device by switching a drivevoltage applied across a pixel liquid crystal between positive andnegative for application based on the input video signal; and means(suppression unit) for suppressing a polarization phenomenon in thedisplay device.

As described above, in the first aspect, the echo phenomenon occurringwhen moving images are displayed can be suppressed by preventing thepolarization phenomenon in the display device.

According to a second aspect, in accordance with the first aspect, themeans for suppressing the polarization phenomenon is an adjustment meansfor adjusting the drive voltage by correcting the input video signal orthe drive voltage so that absolute values of the drive voltage betweenpositive and negative in at least any two successive frames becomecloser.

As described above, in the second aspect, the drive voltage can bebetter balanced between positive and negative by adjusting absolutevalues of the positive and negative drive voltages to be closer. As aresult, ions are not easily distributed unevenly and polarization hardlyoccurs in the insulator film, thereby suppressing the echo phenomenon.

According to a third aspect, in accordance with the second aspect, whenan absolute value of the drive voltage in an n-th frame of a signalbeing normal to the input video signal is different from an absolutevalue of the drive voltage in an n+1-th frame or in an n−1-th frame, theadjustment means adjusts the drive voltage of any one of the n+1-thframe, the n−1-th frame, or the n-th frame.

As described above, in the third aspect, since the drive voltage isadjusted by referring to both a preceding frame and a subsequent frame,adjustment of the drive voltage can be done more optimally.

According to a fourth aspect, in accordance with the third aspect, theadjustment means adjusts the drive voltage of both the n+1-th frame andthe n−1-th frame.

As described above, in the fourth aspect, both of the preceding frameand the subsequent frame are subjected to drive voltage adjustment.Therefore, such adjustment can be done more optimally depending on thevideo signal, and as a result, the resultantly corrected moving imageslook smoother.

According to a fifth aspect, in accordance with the third aspect, whenadjusting the drive voltage, the adjustment means holds a maximum valueor a minimum value.

As described above, in the fifth aspect, by leaving the maximum value orthe minimum value as it is without correcting the same, the originalvideo signal can retain its contrast.

According to a sixth aspect, in accordance with the third aspect, whenadjusting the drive voltage, the adjusting means holds a sum of theabsolute values or a square sum of the absolute values of the n+1-thframe, the n−1-th frame, and the n-th frame.

As described above, in the sixth aspect, by equalizing, in terms oftime, average values of the absolute values of the drive voltage beforeand after correction, correction can be done with little influence, andthe resultant images look smoother.

According to a seventh aspect, in accordance with the second aspect, ifthe polarity of the drive voltage applied to the two successive framesis different from each other, the adjustment means adjusts the drivevoltage to make a difference of the absolute values of the drive voltageto be ½ of a maximum drive voltage or smaller.

As described above, in the seventh aspect, the drive voltage iscorrected so as to be in balance between positive and negative, wherebyecho phenomenon can be restrained to a greater degree.

According to an eighth aspect, in accordance with the seventh aspect,the adjustment means adjusts the drive voltage to make the difference ofthe absolute values of the drive voltage to be 1/10 of the maximum drivevoltage or smaller.

As described above, in the eighth aspect, the drive voltage is correctedso as to be in much better balance between positive and negative, andthus, the echo phenomenon can be suppressed to such a level as not to beperceivable by the naked eye.

According to a ninth aspect, in accordance with the eighth aspect, theadjustment means adjusts the drive voltage when, prior to adjustment,the difference of the absolute values of the drive voltage exceeds 1/10of the maximum drive voltage.

As described above, in the ninth aspect, the drive voltage can becorrected only when the echo phenomenon is to be observed by the nakedeye.

According to a tenth aspect, in accordance with the first aspect, thedrive means divides one vertical scanning period of the input videosignal into a first sub period and a second sub period, and the drivevoltage applied to the sub periods is different in polarity.

As described above, in the tenth aspect, by adjustably balancing thedrive voltage between positive and negative on the basis of a verticalscanning period, the echo phenomenon can be restrained.

According to an eleventh aspect, in accordance with the tenth aspect,the drive means outputs the same video signal in the first sub periodand the second sub period.

As described above, in the eleventh aspect, the drive voltage can beadjusted only by switching the polarity of the drive voltage between thefirst sub period and the second sub period. Therefore, the structure canbe simplified.

According to a twelfth aspect, in accordance with the tenth aspect, thefirst sub period and the second sub period are the same in length.

As described above, in the twelfth aspect, the drive voltage can beadjusted only by simply increasing the signal speed. Therefore, thestructure can be simplified.

According to a thirteenth aspect, in accordance with the tenth aspect,the first sub period and the second sub period are not the same inlength.

As described above, in the thirteenth aspect, adjustment of the drivevoltage can become possible by considering the speed of the ions movingin the liquid crystal, for example.

According to a fourteenth aspect, in accordance with the tenth aspect,the drive means (unit) includes division means for dividing the onevertical scanning period of the input video signal into the first subperiod and the second sub period.

As described above, in the fourteenth aspect, each vertical scanningperiod of the input video signal can be divided, for output, into thefirst sub period and the second sub period. Therefore, the drive voltagecan be adjusted only by switching the polarity of these signals.

According to a fifteenth aspect, in accordance with the fourteenthaspect, the division means includes means for temporarily storing theinput video signal.

As described above, in the fifteenth aspect, even with high-speed drive,the reliability will not be lowered through division.

According to a sixteenth aspect, in accordance with the fourteenthaspect, the division means includes means for delaying the input videosignal by a length of time equal to or shorter than the one verticalscanning period.

As described above, in the sixteenth aspect, the division means can berealized at lower cost.

According to a seventeenth aspect, in accordance with the fourteenthaspect, conversion means (unit) is further comprised for converting theinput video signal into a data display signal for driving the displaydevice, wherein in a process for converting the input video signal intothe data display signal, the conversion means divides the one verticalscanning period of the input video signal into the first sub period andthe second sub period.

As described above, in the seventeenth aspect, the number of requiredconstituents can be reduced, and thus, the liquid crystal displayapparatus can be realized at relatively low cost.

According to an eighteenth aspect, in accordance with the first aspect,the drive means divides one vertical scanning period of the input videosignal into a first sub period and a second sub period, outputs theinput video signal in the first sub period, and outputs a compensationsignal in the second sub period.

As described above, in the eighteenth aspect, a compensation signalwhich is irrelevant to the input video signal which affects imagedisplay not that much is inserted to each vertical scanning period. Inthis manner, uneven ion distribution is eased, and the echo phenomenoncan be restrained.

According to a nineteenth aspect, in accordance with the eighteenthaspect, he second sub period is shorter than the first sub period.

As described above, in the nineteenth aspect, the screen can beprevented from being lowered in brightness due to the compensationsignal being inserted thereto.

According to a twentieth aspect, in accordance with the eighteenthaspect, the drive voltage in the second sub period is a voltage of apedestal level or lower when the display device is a normally blacktype, and the drive voltage is a voltage of the pedestal level or higherwhen the display device is a normally white type.

As described above, in the twentieth aspect, uneven ion distribution canbe cancelled sooner without, nearly, affecting image display, wherebyresultant effects by such improvement can be enhanced.

According to a twenty-first aspect, in accordance with the twentiethaspect, the display device is a normally black type, and the drivevoltage in the second sub period is 0V.

As described above, in the twenty-first aspect, uneven ion distributioncan be cancelled sooner without, nearly, affecting image display,whereby resultant effects by such improvement can be enhanced.

According to a twenty-second aspect, in accordance with the eighteenthaspect, the drive voltage in the second sub period is applied to aplurality of scanning lines at one time.

As described above, in the twenty-second aspect, the time that is takenfor scanning can be shortened by reducing the scanning time that istaken for writing of the compensation signal.

According to a twenty-third aspect, in accordance with the first aspect,the drive means scans, in an n-th frame, odd-numbered scanning lines fora data signal and even-numbered scanning lines for a compensationsignal, and scans, in an n+1-th frame, the odd-numbered scanning linesfor the compensation signal, and the even-numbered lines for the datasignal.

As described above, in the twenty-third aspect, by inserting acompensation signal to all of the pixels on a frame basis, uneven iondistribution can be eased, and the echo phenomenon can be suppressed.Further, the timing for displaying the compensation signal may beshifted by one frame depending on whether the scanning line isodd-numbered or even-numbered. This prevents the screen from gettingblackened in its entirely for every other frame due to the compensationsignal, for example. Moreover, when the video signal is an interlacesignal, there is no need to convert it to a progressive signal.Therefore, the image display apparatus can be realized at relatively lowcost.

According to a twenty-fourth aspect, in accordance with the firstaspect, during one vertical scanning period of the input video signal,after sequentially scanning either one of odd-numbered scanning linesand even-numbered scanning lines, the drive means sequentially scans thescanning line.

As described above, in the twenty-fourth aspect, since a periodicalchange in brightness which is a cause of the echo phenomenon is shiftedby a half period for any adjacent scanning lines, the echo phenomenondoes not become perceivable.

According to a twenty-fifth aspect, in accordance with the first aspect,the drive means applies the drive voltage of the same polarity withoutpolarity reverse to at least any two successive frames.

As described above, in the twenty-fifth aspect, the drive voltage iscorrected to be in better balance between positive and negative, andthus, the echo phenomenon can be restrained.

According to a twenty-sixth aspect, in accordance with the twenty-fifthaspect, the drive means applies such a drive voltage that alternates thepolarity for every two frames.

As described above, in the twenty-sixth aspect, the drive voltage iscorrected to be in better balance between positive and negative, andthus, the echo phenomenon can be restrained.

According to a twenty-seventh aspect, in accordance with thetwenty-fifth aspect, the drive means applies the drive voltage of thesame polarity between any two successive frames once for every n frames.

As described above, in the twenty-seventh aspect, the drive voltage iscorrected to be in better balance between positive and negative, andthus, the echo phenomenon can be restrained.

According to a twenty-eighth aspect, in accordance with the firstaspect, the display device includes liquid crystal, and an orientationfilm, where a combination of the liquid crystal and the orientation filmis a combination to make a voltage holding ratio 98% or higher.

As described above, in the twenty-eighth aspect, the echo phenomenon canbe prevented from occurring.

According to a twenty-ninth aspect, in accordance with the first aspect,the display device includes liquid crystal, and an orientation film,where as the means for suppressing the polarity phenomenon, the liquidcrystal includes 1wt % or less of a CN (cyano group) compound, and theorientation film does not include a high polymer whose conjugated lengthis seven atoms or more.

As described above, in the twenty-ninth aspect, the echo phenomenon canbe prevented from occurring.

According to a thirtieth aspect, in accordance with the first aspect,the display device includes liquid crystal, an orientation film, and apixel electrode and a common electrode for applying the voltage to theliquid crystal, where at least a part of the pixel electrode and thecommon electrode applies the voltage to the liquid crystal only via theorientation film.

As described above, in the thirtieth aspect, electric charge which isthe cause of the echo phenomenon becomes easily absorbed into theelectrode. Accordingly, the echo phenomenon can be prevented fromoccurring.

According to a thirty-first aspect, in any one of the first to thirtiethaspects, the display device includes liquid crystal, and an electrodefor applying the voltage to the liquid crystal, where a part of theliquid crystal is driven in a state that there is none of the electrodein the vicinity thereof.

As described above, in the thirty-first aspect, if the liquid crystalhas any region including no electrode in the vicinity as in the regionbetween electrodes in the IPS-type liquid crystal panel, for example,electric charge which is a cause of the echo phenomenon may stay withoutbeing absorbed into the electrodes. Even if these display devices are tobe used, the echo phenomenon can be successfully suppressed.

According to a thirty-second aspect, in accordance with any one of thefirst to thirtieth aspects, the display device includes liquid crystal,and a pixel electrode and a common electrode for applying the voltage tothe liquid crystal, where the liquid crystal is driven by an electricfield which is generated between the pixel electrode and the commonelectrode, and is almost parallel to a substrate.

As described above, in the thirty-second aspect, the echo phenomenon canbe successfully suppressed even with the IPS-type liquid crystal panelin which electric charge which is a cause of the echo phenomenon maystay without being absorbed into the electrodes.

According to a thirty-third aspect, in accordance with any one of thefirst to twenty-seventh aspects, the display device is made of aplurality of materials, where at least one of the materials is amaterial with which an echo phenomenon easily occurs.

As described above, in the thirty-third aspect, the echo phenomenon canbe successfully suppressed even with the display device being made of amaterial with which the echo phenomenon easily occurs.

A thirty-fourth aspect is directed to an image display method fordisplaying an image by driving a display device based on an input videosignal. The image display method of the thirty-fourth aspect comprises:a drive step of driving the display device by switching a drive voltagebetween positive and negative for application based on the input videosignal; and an adjusting step of adjusting the drive voltage bycorrecting the input video signal or the drive voltage so that absolutevalues of the drive voltage between positive and negative become closerin at least any two successive frames.

As described above, in the thirty-fourth aspect, the drive voltage canbe better balanced between positive and negative by adjusting absolutevalues of the positive and negative drive voltages to be closer. As aresult, ions are not easily distributed unevenly and polarization hardlyoccurs in the insulator film, thereby suppressing the echo phenomenon.

A thirty-fifth aspect is directed to an image display method fordisplaying an image by driving a display device based on an input videosignal. The image display method of the thirty-fifth aspect comprises adrive step of driving the display device by switching a drive voltagebetween positive and negative for application based on the input videosignal. The drive step divides one vertical scanning period of the inputvideo signal into a first sub period and a second sub period, and thedrive voltage applied to the sub periods is different in polarity.

As described above, in the thirty-fifth aspect, by adjustably balancingthe drive voltage between positive and negative on the basis of avertical scanning period, the echo phenomenon can be restrained.

A thirty-sixth aspect is directed to an image display method fordisplaying an image by driving a display device based on an input videosignal. The image display method of the thirty-sixth aspect comprises adrive step of driving the display device by switching a drive voltagebetween positive and negative for application based on the input videosignal. The drive step divides one vertical scanning period of the inputvideo signal into a first sub period and a second sub period, andoutputs the input video signal in the first sub period, outputs acompensation signal in the second sub period.

As described above, in the thirty-sixth aspect, a compensation signalwhich is irrelevant to the input video signal and which does not affectimage display that much is inserted to each vertical scanning period. Inthis manner, uneven ion distribution is eased, and the echo phenomenoncan be restrained. A thirty-seventh aspect is directed to an imagedisplay method for displaying an image by driving a display device basedon an input video signal. The image display method of the thirty-seventhaspect comprises a drive step of driving the display device by switchinga drive voltage between positive and negative for application based onthe input video signal. The drive step scans, in an n-th frame,odd-numbered scanning lines for a data signal and even-numbered scanninglines for a compensation signal, and scans, in an n+1-th frame, theodd-numbered scanning lines for the compensation signal and theeven-numbered lines for the data signal.

As described above, in the thirty-seventh aspect, by inserting acompensation signal to all of the pixels on a frame basis, uneven iondistribution can be eased, and the echo phenomenon can be suppressed.Further, the timing for displaying the compensation signal maybe shiftedby one frame depending on whether the scanning line is odd-numbered oreven-numbered. This prevents the screen from getting blackened in itsentirety for every other frame due to the compensation signal, forexample. Moreover, when the video signal is an interlace signal, thereis no need to convert it to a progressive signal. Therefore, the imagedisplay apparatus can be realized at relatively low cost.

A thirty-eighth aspect is directed to an image display method fordisplaying an image by driving a display device based on an input videosignal. The image display method of the thirty-eighth aspect comprises adrive step of driving the display device by switching a drive voltagebetween positive and negative for application based on the input videosignal. During one vertical scanning period of the input video signal,after sequentially scanning either one of the odd-numbered scanninglines and the even-numbered scanning lines, the drive step sequentiallyscans the scanning line.

As described above, in the thirty-eighth aspect, since a periodicalchange in brightness which is a cause of the echo phenomenon is shiftedby a half period for any adjacent scanning lines, the echo phenomenondoes not become perceivable.

A thirty-ninth aspect is directed to an image display method fordisplaying an image by driving a display device based on an input videosignal. The image display method of the thirty-ninth aspect comprises adrive step of driving the display device by switching a drive voltagebetween positive and negative for application based on the input videosignal. The drive step applies the drive voltage of the same polaritywithout polarity reverse to at least any two successive frames.

As described above, in the thirty-ninth aspect, the drive voltage iscorrected to be in better balance between positive and negative, andthus the echo phenomenon can be restrained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a top view of the structure of a liquid crystal panel 108.

FIG. 3 are section views of the structure of the liquid crystal panel108.

FIG. 4 is a diagram showing waveforms of a voltage to be applied to apixel.

FIG. 5 is a diagram showing waveforms of a voltage to be applied to apixel.

FIG. 6 is a block diagram showing the structure of a modificationexample of the first embodiment.

FIG. 7 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a second embodiment of the presentinvention.

FIG. 8 is a block diagram showing the structure of a double-speedcircuit 112.

FIG. 9 shows timing charts of the operation of the double-speed circuit112.

FIG. 10 is a diagram showing waveforms of a voltage to be applied to apixel.

FIG. 11 is a block diagram showing the structure of a modificationexample of the second embodiment.

FIG. 12 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a third embodiment of the presentinvention.

FIG. 13 is a diagram showing the structure of a liquid crystal panel132.

FIG. 14 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a fourth embodiment of the presentinvention.

FIG. 15 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a fifth embodiment of the presentinvention.

FIG. 16 is a diagram showing an exemplary structure of a liquid crystalpanel.

FIG. 17 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a sixth embodiment of the presentinvention.

FIG. 18 is a diagram showing waveforms of a voltage to be applied to apixel.

FIGS. 19( a) and (b) are diagrams showing the relationship betweenbrightness and voltage in a liquid crystal panel in an NB mode or an NWmode.

FIG. 20 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a seventh embodiment of the presentinvention.

FIG. 21 is a diagram showing waveforms of a voltage to be applied to apixel.

FIG. 22 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to an eighth embodiment of the presentinvention.

FIG. 23 is a diagram showing the change in brightness of any two pixelsadjacent to each other.

FIG. 24 is a block diagram showing the structure of a liquid crystaldisplay apparatus according to a ninth embodiment of the presentinvention.

FIG. 25 is a diagram showing waveforms of a voltage to be applied to apixel.

FIG. 26 is a top view of the structure of a liquid crystal panelaccording to a tenth embodiment of the present invention.

FIGS. 27( a) and (b) are section views of the liquid crystal panel ofthe tenth embodiment.

FIG. 28 is a diagram illustrating how an echo phenomenon occurs in theliquid crystal panel 108.

FIG. 29 is a diagram showing whether the echo phenomenon occurs underwhat condition.

FIG. 30 is a diagram showing whether or not the echo phenomenon occursin the liquid crystal panel varying in structure according to aneleventh embodiment of the present invention.

FIG. 31 is a block diagram showing the structure of a conventionalliquid crystal display apparatus.

FIG. 32 is a diagram showing waveforms of a drive voltage when a testpattern of still images showing gray, white, and gray in the horizontaldirection is displayed under a conventional alternating-current drivemethod.

FIGS. 33( a) and (b) are diagrams showing the electrode structure of anIPS-type liquid crystal panel.

FIG. 34 is a diagram showing drive voltage waveforms, in theconventional liquid crystal display apparatus including the IPS-typeliquid crystal panel, in a case where a test pattern of white, gray, andwhite is moved rightward by two pixels on a frame basis.

FIGS. 35( a) and (b) are diagrams showing an electrode voltage of afocusing pixel A shown in FIG. 34, and an electric field relating to thefocusing pixel A.

FIG. 36 is a diagram showing polarization that occurs in an insulationfilm and uneven distribution of ions in the liquid crystal as a resultof DC voltage application in the IPS-type liquid crystal panel, (−) to acommon electrode 921, and (+) to a drain electrode 922.

FIG. 37 is a diagram showing an exemplary image display when a testpattern is moved leftward in the conventional liquid crystal displayapparatus.

FIG. 38 is a diagram for illustrating how echo streaks occur in theconventional liquid crystal display apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention are described below byreferring to the accompanying drawings.

First Embodiment

In a first embodiment of the present invention, the above-described echophenomenon is prevented by correcting absolute values of a drive voltagewith which a liquid crystal panel is operated under AC drive, wherebythe drive voltage is adjusted so as to balance between positive andnegative. FIG. 1 shows the structure of a liquid crystal displayapparatus according to the first embodiment of the present invention.The liquid crystal display apparatus includes an echo suppressioncircuit 100, a controller 102, a source driver 104, a gate driver 106,and a liquid crystal panel 108.

FIG. 2, and FIG. 3( a) to FIG. 3( c) show the structure of the liquidcrystal panel 108. FIG. 2 is a top view of a unit pixel part of theliquid crystal panel 108. FIG. 3( a) to FIG. 3( c) show, respectively, asection view along A—A of FIG. 2, a section view along B—B, and asection view along C—C. The structure of the liquid crystal panel 108 isdescribed below by referring to these drawings. Here, the liquid crystalpanel 108 is a general type as an IPS-type liquid crystal panel.

On a glass substrate 1, as metal leads, a video signal line (sourceline) 7 and a scanning signal line (gate line) 4 are placed in a matrix,and at each intersection point thereon, a TFT (Thin Film Transistor) isformed as a switching element. On the glass substrate 1, a gateelectrode 4 and common electrodes 5 and 6 made of metal such as Al areformed simultaneously. In the case where the gate electrode 4 and thecommon electrodes 5 and 6 are made of Al, the displaying area isentirely subjected to an anodic oxidation process so as to effectivelyprevent shorts between layers at a part where leads are intersected.Next, a first insulator film 20 (interlayer insulator film) is formed,and then a semiconductor film (amorphous silicon layer) 40 and aprotection layer of the transistor are sequentially formed.

Then, a contact hole having the first insulator film removed therefromis formed in a part not including the display part but being in thevicinity thereof. As a result, contact with the lead part to be formednext becomes possible. Next, by using metal such as Al and Ti, thesignal lead (source line) 7, a drain line 14, and pixel electrodes 8 and9 are formed. In order to protect the TFT formed as described above, asecond insulator film (passivation film) 22 is formed by using SiNx. Thesecond insulator film 22 is formed so as to cover a pixel part in itsentirety. As to another part except for the display part, the secondinsulator film is removed therefrom for such a reason that a terminalpart is located there for application of electrical signals.

The resultant array substrate and a color filter substrate 16 are bothprovided with an orientation film 30 so that liquid crystal 3 issandwiched thereby, generating the liquid crystal panel 108. Here, theliquid crystal 3 includes, as does the general IPS-type liquid crystalpanel, a compound including CN group.

Described below is the operation in the first embodiment. To the liquidcrystal display apparatus, a video signal, and a horizontal/verticalsynchronizing signal are to be inputted. The echo suppression circuit100 corrects the incoming video signal in a manner as will be describedlater to suppress an echo phenomenon which occurs when a video signalincluding moving images is displayed on the liquid crystal display 108.The controller 102 operates the liquid crystal panel with AC drivethrough the source driver 104 and the gate driver 106 based on the videosignal thus corrected by the echo suppression circuit 100, for example.

Described next is the operation of the echo suppression circuit 100.Focusing on one pixel, in the conventional drive method, a voltage suchas the one shown in FIG. 4( a) is applied to the liquid crystal. In thiscase, the voltage to be applied to the liquid crystal is out of balancebetween positive and negative, and therefore the above described echophenomenon occurs in streaks. In the present embodiment, on the otherhand, the video signal is corrected by the echo suppression circuit 100in such a manner as to balance, between positive and negative, thevoltage to be applied to the liquid crystal. Accordingly, as shown inFIG. 4( b), FIG. 4( c), or FIG. 4( d), the voltage which is in balancebetween positive and negative can be applied to the liquid crystal.

The echo suppression circuit 100 divides the video signal into a pair oftwo successive frames, and for every pair, compares these two frames interms of signal level. Then, if these frames are determined as being notthe same in signal level, the signal is corrected so as to be the samein signal level between these two frames. At this time, for example, thehigher signal level may be used as a correction basis, or the lowersignal level may be used as the correction basis. Alternatively, anaverage signal level may be taken between these two frames to use suchaverage signal as the correction basis, or any other levels may be usedas the correction basis. FIG. 4( b) shows the drive voltage to theliquid crystal with the correction basis of the lower level, FIG. 4( c)shows the drive voltage to the liquid crystal with the correction basisof the higher level, and FIG. 4( d) shows the drive voltage to theliquid crystal with the correction basis of the average level.

As shown in FIG. 4( b) or FIG. 4( c), by using the higher level(maximum) or the lower level (minimum) of the compared frames as a basisof correcting the signal level thereof, contrast between bright and darkis emphasized to a greater degree than in the case where the averagelevel is taken between the frames to use it as the basis as shown inFIG. 4( d). Accordingly, there is such an advantage that the time-baserepresentation of videos will become well-defined. On the other hand, byusing the average level of the frames as the basis as shown in FIG. 4(d), a gradual temporal change in tone can be achieved so that the movingimages can be advantageously displayed more smoothly. Also, by using theaverage level of the frames as the basis, a change occurring to the sumof the absolute values of the drive voltage is smaller a compared withthe case where no correction is performed. Since human eyes hardlyperceive a change in brightness caused by correction in such a manner,there is an advantage that correction can be performed without that muchinfluence.

Alternatively, correction may always be performed on the basis of eitherthe maximum value or the minimum value, or the basis may be switchedtherebetween depending on the situation. In the case where correction isalways performed on the basis of either the maximum value or the minimumvalue, the circuit structure can be advantageously simplified.Alternatively, as an example, a reference intermediate tone level (e.g.,a value which is exactly in the middle between the maximum tone leveland the minimum tone level of the display apparatus) may be provided.According to the level of the video signal, correction may be performedbased on either the maximum value or the minimum value, whichever isfarther from this reference intermediate tone level. Through suchcorrection, the resultant signal turns out to be brighter or darker thanthe intermediate tone so that videos are advantageously well-definedeasily.

Here, in the present embodiment, the drive voltage level is firstcompared for every two frames, and then the video signal is corrected soas to equalize the signal level between those two frames. This is notrestrictive, and the number of frames used as the basis of correctingthe video signal to balance the drive voltage level between positive andnegative may be three or four, for example. To be specific, as analternative to the conventional drive method shown in FIG. 5( a), thevideo signal may be corrected on the basis of three frames as shown inFIG. 5( b) to balance the drive voltage level between positive andnegative. By increasing the number of frames to be compared as such,adjustment for incoming video signals can be done more flexibly ascompared with the case where adjustably balancing between positive andnegative for every two frames as in the present embodiment, and thusthere is an advantage in that correction can be performed without thatmuch influence. Assuming here that the drive voltage is adjustablybalanced between positive and negative on a frame basis of three or moreframes. If the balance adjustment between positive and negative is donenot by correcting any frame signal at least a maximum or minimum insignal level among a plurality of frames to be used as the basis but byusing any other frame signal, any image characteristic point (e.g.,conspicuously bright part or dark part) is prevented from being lost orbecoming inconspicuous as a result of correction. Therefore, imagedegradation due to correction can be suppressed. Further, through suchcorrection as not changing the sum of the absolute values of the drivevoltage on the basis of frames, correction can be performed, withoutthat much influence, so as not to make a change in brightness causedthereby perceivable that much.

Here, the brightness of the liquid crystal display element isproportional to about the square of the drive voltage. Generally, in thedisplay apparatus, the relationship between a tone signal and thebrightness is also proportional to about the square of the drivevoltage. Accordingly, to perform correction in a manner so as not tochange the sum of the absolute values of the brightness with moreaccuracy, correction may be performed so as not to change the sum of thesquare of the drive voltage. If this is the case, correction can beperformed, without that much influence, so as to make a change inbrightness caused thereby unperceivable. Note here that, retaining thesum of the absolute values as mentioned in the above is easier forrealization with circuits, for example.

Here, those various correction methods described above may be changed asappropriate depending on the situation. For example, those correctionmethods exemplified by FIG. 4( b), FIG. 4( c), and FIG. 4( d) may be allused for correction by switching thereamong depending on what the videoscenes are for.

Note here that, although many of the correction methods are exemplifiedin the above, what is important to suppress the echo phenomenon isperforming correction in a manner that the drive voltage can be balancedbetter between positive and negative. In consideration thereof, anyother arbitrary correction methods which can achieve that are surelyapplicable as well. Moreover, the video signal is subjected tocorrection to balance the drive voltage between positive and negative inthe present embodiment. This is not the only possibility, and the drivevoltage to be outputted from the controller 102 may be subjected tocorrection.

In order to operate as such, although not shown, the echo suppressioncircuit 100 needs to include a memory for storing video signals of aplurality of fields, a computation part for comparing the signal levelamong those fields, and a circuit for controlling the memory andcomputation part. As to its circuit structure, however, there is nospecific limitation, although a low-priced simple circuit is preferable.

Further, in the present embodiment, as shown in FIG. 1, the echosuppression circuit 100 is structured separately from otherconstituents, but this is not restrictive. As shown in FIG. 6, forexample, the echo suppression circuit 100 may be functionallyincorporated, in terms of process, into a TV-LCD conversion circuit 110for inputting TV signals into the liquid crystal panel 108, or a pixelconversion circuit 110 for resolution conversion, for example. If thisis the case, the number of constituents that are required, for theliquid crystal display apparatus can be reduced, and thus the liquidcrystal display apparatus can be realized at relatively low cost.

Still further, in the present embodiment, correction is performed forevery pair of two frames in such a manner as to equalize the absolutevalues of the drive voltage. This is not restrictive, and bringing theabsolute values closer to each other as much as possible can also reducethe echo phenomenon. Specifically, the echo phenomenon provably showsconsiderable improvement when correction is performed so as to keep adifference of absolute values a half of the maximum drive voltage orsmaller. Furthermore, when correction is performed so as to keep thedifference of absolute values smaller than a tenth thereof, the echophenomenon is successfully suppressed to a level that is unobservable bythe naked eye. These results show that limitedly performing correctiononly when any two successive frames show a difference of absolute valuesof the drive voltage being a tenth of the maximum drive voltage orlarger will lead to more efficiency at the time of correction.

Still further, in the present embodiment, correction is performed so asto balance the drive voltage between positive and negative on the basisof a plurality of frames. This is not restrictive, and correction may beperformed, for example, sequentially through frame comparison, one framewith another frame precedent thereto. If this is the case, with respectto any frame to be corrected, considering not only a frame precedentthereto but also another frame subsequent thereto may give moreflexibility to the amount of frame correction to be determined.

As described in the foregoing, according to the first embodiment, bycorrecting video signals composed of frames or a drive voltage on thebasis of a plurality of frames in such a manner as to balance, betweenpositive and negative, the drive voltage to liquid crystal, an echophenomenon can be prevented when moving images are displayed.

Second Embodiment

In a second embodiment of the present invention, the above-describedecho phenomenon is prevented by alternating the polarity of a drivevoltage with which a liquid crystal panel is driven, whereby the drivevoltage is adjusted so as to balance between positive and negative foreach frame. FIG. 7 shows the structure of a liquid crystal displayapparatus according to the second embodiment of the present invention.In FIG. 7, the liquid crystal display apparatus includes a double-speedcircuit 112, the controller 102, the source driver 104, the gate driver106, and the liquid crystal panel 108. The operation of the liquidcrystal display apparatus in the present embodiment is described below.

The double-speed circuit 112 converts, based on a incoming video signaland a synchronizing signal to be inputted, a frame frequency of theincoming video signal to be doubled. The controller 102 performs timingcontrol of the source driver 104 and the gate driver 106, and AD drivecontrol of the liquid crystal panel 108 based on an output from thedouble-speed circuit 112. The structure and the operation of thedouble-speed circuit 112 is described in more detail below.

FIG. 8 is a block diagram showing an exemplary structure of thedouble-speed circuit 112 of the present embodiment. In FIG. 8, thedouble-speed circuit 112 includes a dual port RAM 114, a writing addresscontrol circuit 116, a reading address control circuit 118, and asynchronizing signal control circuit 120.

The dual port RAM 114 is a random access memory in which a writingaddress/data port and a reading address/data port are providedseparately, and thus, writing and reading can be performed separately.An incoming video signal is inputted into the writing port of the dualport RAM 114, and according to a writing address coming from the writingaddress control circuit 116, is written into the dual port RAM 114. Thevideo signal data that is written into the dual port RAM 114 is thenread and outputted by the dual port RAM 114 in accordance with a readingaddress coming from the reading address control circuit 118. In responseto an input vertical synchronizing signal, an input horizontalsynchronizing signal, and an input clock, the synchronizing signalcontrol circuit 120 controls both the writing address control circuit116 and the reading address control circuit 118, and also outputs anoutput vertical synchronizing signal, an output horizontal synchronizingsignal, and an output clock which are all converted in frequency to bedoubled with respect to the inputs. The operation of the double-speedcircuit 112 is described more specifically below by referring to FIG. 9.

FIG. 9 is a set of timing charts showing the operation of thedouble-speed circuit 112 of the present embodiment. In FIG. 9, thelateral axis indicates time, and in the vertical direction, a signalsize is indicated for the input/output vertical synchronizing signalsand an address is indicated for the writing/reading addresses. As shownin FIG. 9, the writing address outputted from the writing addresscontrol circuit 116 is counted up responding to a clock input, and resetto a vertical blanking interval in response to the input verticalsynchronizing signal. The writing data is an input video signal, and aframe of the input video signal is stored on the dual port RAM 114. Thereading address is, on the other hand, counted up in response to anoutput clock which is the one obtained by multiplying the input clock bya PLL, for example, and is reset by a frequency which is the two-fold ofthe frequency of the input vertical synchronizing signal. If the timingof resetting the count of the reading address is synchronized with thetiming of resetting the count of the writing address, and if the countreset timing of the reading address coincides with the count resettiming of the writing address once to twice, any images showingdiscontinuity caused by writing and reading getting out of order can beremoved to the outside of the effective screen. As such, thedouble-speed circuit 112 is realized from which one frame of the inputvideo signal is outputted at the doubled frame frequency twice in a row.

FIG. 10( a) is a diagram showing a change occurring to the drive voltageto the liquid crystal of a focusing pixel when the liquid crystal isdriven under the conventional drive method. FIG. 10(b) is, on the otherhand, a diagram showing a change occurring to the drive voltage to theliquid crystal of the focusing pixel when the liquid crystal is drivenunder the drive method of the present embodiment. As shown in FIG. 10(b), in the present embodiment, the drive voltage to the liquid crystalis balanced between positive and negative on a frame basis. That is, thepolarity of an electric field applied to the liquid crystal of the pixelA is in balance between positive and negative even during patternpassing and thereafter. Therefore, the problem of uneven iondistribution in the liquid crystal panel can be prevented in a periodfor vertical scanning the video signal which is originally inputted. Asa result, there occurs no tail echo when displaying moving images, whichis the problem occurring in the conventional liquid crystal displayapparatus.

As such, according to the present embodiment, the incoming video signalis converted so as to be doubled in frame frequency for operating theliquid crystal panel with AC drive. In more detail, each frame of theoriginal video signal is further divided into two periods (sub frames)of a normal period and a compensation period, the length of which is thesame. The polarity is changed between the normal period and thecompensation period for driving the liquid crystal panel, canceling outthe progress of uneven distribution of the ions for each frame of thevideo signal. In such a manner, regardless of the pattern in which thedisplaying moving images are, the ions will never be distributedunevenly to a further extent, and the insulator film will never bepolarized to a further extent. Accordingly, image degradation as aresult of the echo phenomenon can be restrained.

Here, the above-described liquid crystal panel is presumed to be theIPS-type liquid crystal panel. This is not the only option, and whateverarbitrary liquid crystal panel in which the echo phenomenon is observedcan surely restrain image degradation as a result of the echophenomenon.

Further, the above-described double-speed circuit 112 is structured bythe dual port RAM 114. The double-speed circuit 112 can also be realizedby using a single port RAM to share writing and reading by time.Moreover, the double-speed circuit 112 is not limited in structure assuch, and any other arbitrary structure is surely applicable thereto. Interms of simplicity and low cost, the above described structure isconsidered to be preferable.

Note that, in the present embodiment, the original video signal issimply doubled in frame frequency for operation with the verticalscanning period that is divided into the normal period and thecompensation period being the same in length. This is not restrictive,and the ratio between the normal period and the compensation period maybe arbitrary. Here, if the normal period and the compensation period areset the same in length, the signal needs only to be simply doubled sothat the circuit structure is simplified. It should be noted here thatsome combination of the panel structure, the liquid crystal material,and the orientation film, for example, may vary ion behavior dependingon whether the frame is driven by the positive drive voltage ornegative. Therefore, it is preferable that the length ratio betweenthose two sub frames is optimally set in consideration of the ionbehavior.

Here, in the present embodiment, the drive voltage is sequentiallyswitched between positive and negative in order depending on which frameis in the normal period or in the compensation period, and thisswitching order is not restrictive. For example, is the switching ordermay so set that the normal period of the n-th frame is driven by thepositive voltage and the compensation period thereof by the negativevoltage, and the normal period of the n+1-th frame is driven by thenegative voltage and the compensation period thereof by the positivevoltage. If this is the case, the compensation period of the n-th frameand the normal period of the n+1-th frame become the same in polarity asthe drive voltage. Therefore, the compensation voltage of the precedingframe will have precharge effects so that the voltage step at the timeof writing the drive voltage of the normal period is minimal. As aresult, the required level of the charge capacity of the TFT can beadvantageously lowered.

Note that, in the present embodiment, in the double-speed circuit 112,signals are doubled in speed by using a memory which is provided forstoring video signals. This is not restrictive, and speed-doubling maybe carried out by using a delay circuit, for example. If such a delaycircuit is used, the double-speed circuit 112 can be manufactured atlower cost since the memory is not burdened, but at higher speed, thereliability of signals is decreased.

Further, in the present embodiment, the proprietary circuit is used forspeed-doubling. As shown in FIG. 11, for example, the double-speedcircuit 112 may be functionally incorporated into a TV-LCD conversioncircuit 122 for inputting TV signals into the liquid crystal panel, or apixel conversion circuit 122 for resolution conversion, for example. Ifthis is the case, the number of required constituents can be reduced,and thus, the liquid crystal display apparatus can be realized atrelatively low cost.

As described in the foregoing, in the second embodiment, each videosignal is divided into two sub frames for driving the liquid crystal.Thus, signal application to one pixel in one frame needs to be donetwice, making pixel design difficult in the liquid crystal displayapparatus of a higher resolution (XGA or higher). To deal with thisproblem, as a method of enabling high-speed scanning in such a liquidcrystal display apparatus, there is a method of doubling a time takenfor selecting one scanning line, and driving the device in such a mannerthat a half of the time for selecting one scanning line is superposed ona time for selecting the next scanning line, or a method of graduallychanging the capacity Cst, Cgd, or the like, on a pixel basis, forexample. However, even under these techniques, designing the TFT, forexample, becomes more difficult as the resolution gets higher.

Therefore, as shown in more detail below, a third embodiment providessuch a structure for easily enabling the high speed scanning in theliquid crystal display apparatus.

Third Embodiment

FIG. 12 shows the structure of a liquid crystal display apparatusaccording to a third embodiment of the present invention. Here, in FIG.12, any constituent identical to that of FIG. 11 is provided with thesame reference numeral, and is not described again. A controller 124controls a first source driver 126, a second source driver 128, and agate driver 130. FIG. 13 shows the TFT arrangement of a liquid crystalpanel 132. In the present embodiment, as shown in FIG. 13, the number ofdata signal lines is doubled, and pixels for two lines are to be drivenby one scanning line. As a result, the time that is taken for scanningone scanning line can be doubled. In other words, the number of thescanning lines can be reduced to half without lowering the resolution inthe vertical direction. Therefore, even with double-speed drive, thetime that is taken for scanning one scanning line can remain the same asconventional. Here, since the number of the source lines is doubled,wiring density is increased in the part where the source driver isincorporated. To avoid this, the source driver may be divided into two,and provided at two positions as shown in FIG. 13.

Fourth Embodiment

FIG. 14 shows the structure of a liquid crystal display apparatusaccording to a fourth embodiment of the present invention. Here, in FIG.14, any constituent identical to that of FIG. 11 is provided with thesame reference numeral, and is not described again. This liquid crystaldisplay apparatus performs display by switching between a TV videosignal and a PC video signal by using a switching switch 134.Specifically, the TV video signal is doubled in speed at the same timeas it is converted into an LCD video signal in the TV-LCD conversioncircuit/pixel conversion circuit 122. On the other hand, the PC videosignal is not doubled in speed when it is converted into the LCD videosignal in a PC-LCD conversion circuit/pixel conversion circuit 136.

When the PC signal is outputted, a video with lower degree of movementis displayed, and thus, no echo phenomenon is observed. Accordingly, inthe present embodiment, as shown in FIG. 14, when displaying movingimages such as TV/VTR signals, speed-doubling is carried out as in theabove described second embodiment, and the liquid crystal is driventhrough polarity reverse after dividing each video signal into two subframes. When displaying the PC signal, on the other hand, in a mannersimilar to the conventional, the polarity of the data signal is reversedon a frame basis for driving.

As such, according to the fourth embodiment, the liquid crystal displayapparatus is realized in which no echo phenomenon is observed whendisplaying moving images such as TV/VTR signals. Note that the currentTV/VTR signals are interlace signals, and thus, the resolution in thedirection of vertical scanning is ½. Thus, in order to display suchTV/VTR signals, an interlace-progressive (IP) conversion circuit isrequired in the TV-LCD conversion circuit/pixel conversion circuit 122.Accordingly, in the present embodiment, any signal processing relatingto the TV video signals is realized in one circuit, thereby reducing thenumber of constituents and cost. In this regard, the present embodimentis considered to be suitable for a case where video signals of movingimages are interlace signals.

Fifth Embodiment

FIG. 15 shows the structure of a liquid crystal display apparatusaccording to a fifth embodiment of the present invention. Here, in FIG.15, any constituent identical to that of FIG. 14 is provided with thesame reference numeral, and is not described again. This liquid crystaldisplay apparatus performs display by switching between the TV videosignal and the PC video signal by using the switching switch 134.

When the PC signal is outputted, a video with lower degree of movementis displayed, and thus, no echo phenomenon is observed. Accordingly, inthe present embodiment, as shown in FIG. 15, when displaying movingimages such as TV/VTR signals, speed-doubling is carried out as in theabove described second embodiment, and the liquid crystal is driventhrough polarity reverse after dividing each video signal into two subframes. If this is the case, a controller 137 controls the gate driver106 in such a manner as to scan two scanning lines at one time. Withsuch a structure, those two scanning lines which are scanned at the sametime are written with the same signal. Therefore, the resolution in thedirection of the scanning line reduced to ½, while the charge time forone scanning line can remain the same as conventional for scanning.Here, as shown in FIG. 16, also by placing TFTs in a staggeredarrangement with respect to the gate wiring, the fact that theresolution in the direction of the scanning line has been dropped to ½can hardly be perceived by the naked eye. When displaying the PC signal,on the other hand, in a manner similar to the conventional, the polarityof the data signal is reversed on a frame basis for driving.

As such, according to the fifth embodiment, with the conventional TFTcapacity, the PC signal can be displayed without lowering theresolution. Also, when displaying moving images such as TV/VTR signals,the liquid crystal display apparatus is realized in which no echophenomenon is observed. Here, since the current TV/VTR signals areinterlace signals, the resolution in the direction of the verticalscanning lines is ½. Therefore, an interlace-progressive (IP) conversioncircuit is not required in the liquid crystal display apparatus of thepresent invention, successfully lowering the cost. Thus, the presentinvention is considered to be suitable for a case where video signals ofmoving images are interlace signals.

Sixth Embodiment

FIG. 17 shows the structure of a liquid crystal display apparatusaccording to a sixth embodiment of the present invention. Here, in FIG.17, any constituent identical to that of FIG. 1 is provided with thesame reference numeral, and is not described again. In the presentembodiment, in an echo suppression circuit 138, for the purpose ofeasing uneven ion distribution, the video signal is corrected in such amanner as to insert thereto a compensation signal of a black-levelvoltage which hardly affects displaying videos. An exemplary case ofusing a panel of NB mode is described below in a specific manner.

FIG. 18( a) shows an example of voltage application to one pixel underthe conventional drive method. Also, FIG. 18( b) shows another exampleof voltage application to one pixel under a drive method of the presentembodiment. Under the conventional drive method, the video signal isapplied to each frame for the duration of a frame. On the other hand,under the drive method of the present embodiment, each frame is dividedinto two sub frames of a first sub frame and a second sub frame. In thefirst sub frame, a video signal is applied, and a black video signal,i.e., the drive voltage of 0V as a compensation signal for cancelinguneven ion distribution is applied in the second sub frame. Then, on thebasis of a frame (on the basis of two sub frames), the polarity of thevoltage applied to the liquid crystal is reversed. Here, the lengthratio between the first sub frame and the second sub frame is notnecessarily required to be 1:1. The longer the length of the second subframe, the darker the display screen becomes. Therefore, as to thislength, the shorter would be the better.

In the present embodiment, through such operation, uneven iondistribution which occurs as a result of the application of the drivevoltage in the first sub frame can be eased by application of theblack-level voltage in the second sub frame. Accordingly, uneven iondistribution is eased in each frame, resultantly suppressing theoccurrence of the echo phenomenon.

In the present embodiment, the display period for the video signalbecomes shorter than one frame. Generally, the conventional liquidcrystal display apparatus has been a hold-type display device, and as acommon notion, moving images are considered to be easily blurred thereinas compared in a pulse-type display device such as CRTs (ElectricalCommunications Association papers vol. 1. J68B, No. 12 (1985-12)). Underthe drive method of the present embodiment, since each frame isblackened for once as already described, there is an advantage that blurinherent in the hold-type display device hardly occurs.

Here, in the present embodiment, similar to the second embodiment,signal application needs to be done twice to a single pixel on a framebasis. Accordingly, the drive methods in the third embodiment and thefourth embodiment may be combined for further effects.

Note that, in the present embodiment, the black-level voltage applied tothe second sub frame is assumed to be 0V. Actually, however, theblack-level voltage is often not 0V due to the manufacturing reasons ofthe liquid crystal display apparatus. In such a case, as exemplaryvoltage application to one pixel, there may be two manners as shown inFIG. 18( c) and FIG. 18( d). FIG. 18( c) shows a case where the firstsub frame and the second sub frame are the same in polarity, while FIG.18( d) shows a case where the first sub frame and the second sub frameare not the same in polarity. Here, the black-level voltage at this timeis preferably of a pedestal level or lower. Further, to the second subframe, applying the black level which is different in polarity from thatto the first sub frame as shown in FIG. 18( d) is better than applyingthe black level which is the same in polarity from that to the first subframe as shown in FIG. 18( c). This is because, uneven ion distributionis thereby easily eased, and thus, it is considered preferable in viewof suppressing the echo phenomenon.

Here, the voltage of pedestrian level or lower means the voltage showingan absolute value which is farther from white than the original blacksignal. By taking a liquid crystal display apparatus in NB mode as anexample, the relationship of T-V property (brightness-voltage property)will be of such a relationship as the one shown in FIG. 19( a). Avoltage AA shown in FIG. 19( a) is the voltage at the time of displayinga black signal based on the video signal in the normal manner. In theliquid crystal display apparatus in NB mode, the voltage of the pedestallevel or lower means the voltage which is equal to or lower than thevoltage AA. By using the voltage which is lower than the pedestal levelas a compensation signal, the change observed in absolute values betweenthe video signal and the compensation signal becomes larger than thecase where the general black-level voltage (voltage AA) is used. As aresult, uneven ion distribution is cancelled much faster, andaccordingly, the echo phenomenon can be suppressed more effectively.Here, in the case of a liquid crystal display apparatus in NW mode, therelationship of T-V property will be of such a relationship as the oneshown in FIG. 9( b). Accordingly, to achieve the same effects as thecase of NB mode described above, as a compensation signal, the voltagewhich is equal to or higher than the pedestal level, that is, thevoltage which is equal to or higher than the general black-level voltage(voltage BB) may be used.

Further, in the present embodiment, there is a need to write the signalvoltage to each pixel on the basis of a scanning line in the first subframe, but in the second sub frame, the same compensation voltage iswritten into each pixel. Thus, n lines (e.g., four lines) may be writtenwith the compensation signal all at once. If so, the writing period ofthe second sub frame can be reduced to 1/n compared with the writingperiod of the first sub frame. As a result, the charge time for ascanning line to which the signal voltage is written can be longer, andthus, demands for the charging property of the TFT can be eased.

Seventh Embodiment

FIG. 20 shows the structure of a liquid crystal display apparatusaccording to a seventh embodiment of the present invention. Here, inFIG. 20, any constituent identical to that of FIG. 1 is provided withthe same reference numeral, and is not described again. In the presentembodiment, similar to the sixth embodiment described above, in an echosuppression circuit 140, a video signal is corrected so as to beinserted with a compensation signal of a black-level voltage whichhardly affects displaying videos for the purpose of easing uneven iondistribution. An exemplary case of using a panel in NB mode is describedbelow more specifically.

FIG. 21( a) shows an exemplary voltage application to a pixel under theconventional drive method. FIG. 21( b) shows another exemplary voltageapplication to a pixel under a drive method of the present embodiment.Here, in the above-described sixth embodiment, since signal applicationhas to be done twice for a pixel in each frame, TFTs in the pixels andperipheral circuits may be limited in design. In the present embodiment,on the other hand, a signal voltage is applied to a pixel in the firstframe, and a black video signal, i.e., a drive voltage of 0V, as acompensation signal for canceling uneven ion distribution, is applied toa pixel in the second frame. Thereafter, the polarity of the signalvoltage is reversed for every two frames.

In the present embodiment, through such operation, uneven iondistribution which occurs as a result of the application of the drivevoltage in the first frame can be cancelled by application of theblack-level voltage in the second frame. Accordingly, uneven iondistribution is cancelled in two frames, resultantly suppressing theoccurrence of the echo phenomenon.

Here, if the black voltage is applied to all of the pixels on thedisplay screen at the same time, flicker occurs at a frequency half ofthat of the video signal. Thus, in order to prevent this, frame shiftingis taken place in a one-to-one relationship to lines, columns, or dots.Assume here that the video signal is applied to even-numbered lines inan n-th frame, and the black-level signal is applied to odd-numberedlines therein. Then, in an n+1-th frame, the video signal is applied toodd-numbered lines, and the black-level signal is applied toeven-numbered lines. If this is the case, the resolution in one frame ofthe screen is actually reduced to half, but because of the human eyes'accumulated effects, the resolution will be reduced to only about 25%.Presently, since video signals used for TVs and VTRs are often in theinterlace format, with drive of the present invention in accordance withthe respective format of the video signals, an interlace-progressive(IP) conversion circuit is not required, thereby reducing the cost ofthe resultant liquid crystal display apparatus. In this regard, thepresent embodiment is considered to be suitable for a case where inputvideo signals are interlace signals.

Eighth Embodiment

FIG. 22 shows the structure of a liquid crystal display apparatusaccording to an eighth embodiment of the present invention. Here, inFIG. 22, any constituent identical to that of FIG. 1 is provided withthe same reference numeral, and is not described again. In theconventional liquid crystal display apparatus, the scanning lines havebeen sequentially scanned in order line by line. Conversely, in thepresent embodiment, the scanning lines are scanned every other line by acontroller 142.

FIG. 23( a) and FIG. 23( b) show a change in brightness that is observedin, respectively, any two pixels adjacent to each other in response toany Direct-Current component applied thereto. From FIG. 23( a) and FIG.23( b) showing changes in brightness, it is known that the brightnessstarts to periodically vary after the DC voltage application, and theecho phenomenon is observed in both of those pixels.

However, under the drive method of the present embodiment, as shown inFIG. 23( a) and FIG. 23( b), the drive voltage waveform of the liquidcrystal of those adjacent pixels is shifted by a half period. When aperson sees such two pixels, his/her eyes may spatially average those,and recognize that as a change in brightness as shown in FIG. 23( c).Therefore, no echo phenomenon is to be observed.

Ninth Embodiment

FIG. 24 shows the structure of a liquid crystal display apparatusaccording to a ninth embodiment of the present invention. Here, in FIG.24, any constituent identical to that of FIG. 1 is provided with thesame reference numeral, and is not described again. In the conventionalliquid crystal display apparatus, the polarity of the drive voltage isset so as to alternate frame by frame. In the present embodiment, on theother hand, for the purpose of restraining the echo phenomenon, acontroller 144 controls the drive voltage in such a manner that thepolarity does not alternate once for n frames. Here, n is presumably 2or larger.

FIG. 25( a) is a diagram showing a change which is observed in the drivevoltage applied to the liquid crystal of a focusing pixel in a casewhere the liquid crystal is driven under the conventional drive method.FIG. 25( b) is a diagram showing a change which is observed in the drivevoltage of the liquid crystal of the focusing pixel in a case where thepolarity of the drive voltage is not alternated once for every twoframes. FIG. 25( c) is a diagram showing a change which is observed inthe drive voltage applied to the liquid crystal of the focusing pixel ina case where the polarity of the drive voltage is not alternated oncefor every 9 frames.

As such, by setting the polarity of the drive voltage not to alternateonce for n frames, it is confirmed that the echo phenomenon can besuccessfully restrained. This is because, with the conventional AC driveas shown in FIG. 25( a), the ratio between a positive part and anegative part both diagonally shaded therein is about 2:1, and when thepolarity alternates for every two frames as shown in FIG. 25( b), theratio between a positive part and a negative part both diagonally shadedtherein is about 3:2. In view of the comparison result derived by thoseshaded parts, the latter is considered to be better balanced. Further,as shown in FIG. 25( c), the echo phenomenon is also confirmed as beingrestrained when the polarity is set so as not to alternate once for nineframes. In this case, if any one of the nine frames except for thoseshaded in FIG. 25( c) is conspicuously high in voltage as the 5th frameshown in FIG. 25( a), an echo phenomenon occurs due to the voltage ofhigher level but is definitely restrained as compared with the caseunder the conventional AC drive. If such a voltage of higher level isfound in the shaded frame(s) shown in FIG. 25( c), however, an echophenomenon is problematically encouraged to occur as compared with thecase under the conventional AC drive. As shown in FIG. 25( c), however,the shaded parts occupy no more than 2/9 as a whole. Therefore, even ifan input signal pattern shows one frame which is conspicuously high involtage, the probability of the voltage of the higher level applying tothose shaded parts, that is, the successive frames of the same polarityis merely 2/9. Accordingly, in terms of the probability theory, an echophenomenon is successfully suppressed. Here, in FIG. 25( c), thepolarity of the drive voltage does not alternate once for nine frames,but this is not restrictive.

As described in the above, according to the ninth embodiment, there isonly a need to change the timing for polarity reverse of the drivevoltage. Accordingly, the echo phenomenon can be suppressed quiteeasily.

Tenth Embodiment

FIG. 26 shows a top view of a unit pixel of a liquid crystal panelaccording to a tenth embodiment of the present invention. In the presentembodiment, in order to ease uneven ion distribution, a pixel electrodeand a common electrode apply the voltage to the liquid crystal onlythrough an orientation film. FIG. 27( a) is a section view along B—Bshown in FIG. 26. FIG. 27( b) is a section view along A—A shown in FIG.26. With such a structure, according to the present embodiment, iondistribution in the liquid crystal will not become so uneven as comparedwith the general IPS-type liquid crystal panel shown in FIG. 2 and FIG.3. A further description is given below. It should be noted here thatthe structures shown in FIG. 26 and FIG. 27 are not more than a specificexample, and are not restrictive.

On the glass substrate 1 which is a part of the liquid crystal panel, asmetal leads, the video signal line (source line) 7 and the scanningsignal line (gate line) 4 are placed in a matrix. At each intersectionpoint of these signal lines, a TFT (Thin Film Transistor) 15 is formedas a switching element. On the glass substrate 1, the gate electrode 4and the common electrodes 5 and 6 made of metal such as Al are formedsimultaneously. Next, the first insulator film 20 (interlayer insulatorfilm) is formed, and then a semiconductor film (amorphous silicon layer)and a protection layer of the transistor are sequentially formed. Then,the first insulator film 20 is removed from a part not including thedisplay part but being in the vicinity thereof so that contact with thelead part becomes possible. At the same time, the first insulator film20 on the common electrode in the pixel is partially removed, and thus acontact hole is formed.

Then, by using metal such as Al/Ti, another signal lead (source line) 7,the drain line 14, the pixel electrodes 8 and 9, and a second commonelectrode 12 are formed. At this time, the second common electrode 12 iselectrically connected to the first common electrode 5 via the contacthole 10. Here, the first common electrode 5, the second common electrode12, and the contact hole 10 are not specifically limited in placementposition. Here, by placing the first common electrode farther from thegate wiring, shorts between the gate wiring and the common electrodescan be prevented.

In order to protect the TFT formed as described above on the substrate,SiNx is formed as the second insulator film (passivation film) 22. Thesecond insulator film 22 on a terminal part by which an electricalsignal for the part around the display area but not including the sameis applied is removed, and the second insulator film 22 is also removedfrom the rest of the part excluding the TFTs in the pixel.

With such a structure, in the present embodiment, no echo phenomenonoccurs when displaying moving images. As shown in FIG. 27(a) and FIG.27( b), inorganic insulator films over the pixel electrodes and thecommon electrodes have been removed, and thus, the pixel electrodes andthe common electrodes apply the voltage to the liquid crystal via onlythe orientation film. Therefore, even if ions get distributed unevenly,such unevenly distributed ions become nonionic as they are recovered bythe pixel electrodes and the common electrodes.

Note that, in the present embodiment, both of the pixel electrode andthe common electrode are abutting to the liquid crystal only through theorientation film, but this is not restrictive. If at least a part of thepixel electrode and the common electrode is structured so as to applythe voltage to the above-described liquid crystal only through theabove-described orientation film, uneven ion distribution is accordinglyeased and the echo phenomenon is suppressed. That is, such a structurewill do as, in the unit pixel, at least a part of the pixel electrodeand the common electrode are structured so as to apply the voltage tothe liquid crystal only through the orientation film.

Eleventh Embodiment

As an eleventh embodiment of the present invention, the structure of aliquid crystal panel in which ions hardly get unevenly distributed isdescribed.

For reference purpose, FIG. 28( a) and FIG. 28( b) show a change inbrightness when a Direct-Current component is applied to a pixel of thegeneral IPS-type liquid crystal panel 108 shown in FIG. 2, and FIG. 3(a) to FIG. 3( c). FIG. 28( a) shows a voltage level to be applied to thepixel, and FIG. 28( b) shows a change in brightness when a voltageapplied thereto is of the voltage level. In the liquid crystal panel108, the brightness periodically varies as shown in FIG. 28( b), and asa result, an echo phenomenon is observed.

Here, as to the material structuring the liquid crystal panel, as shownin FIG. 29, the echo phenomenon is found to be observed in any of aliquid crystal panel using liquid crystal which includes 1% or more of aCN compound, a liquid crystal panel using a polyimide orientation film(low-resistance orientation film) which includes the following compoundwhose conjugated length is seven or more atoms, a liquid crystal panelusing liquid crystal to which such an additive as ionic compound isadded, and a liquid crystal panel in which the volume of ions in liquidcrystal is increased by irradiating UV ray to the liquid crystal, forexample.

Herein, an IPS-system is exemplified for comparison purpose, but anyother liquid crystal modes may lead to the same phenomenon. Also, thisarray substrate structure is not restrictive, and this structure is nomore than an exemplary one for comparison.

The liquid crystal panel of the present embodiment is the same as theliquid crystal panel 108 shown in FIG. 2 and others except for theliquid crystal and the orientation film material. The liquid crystalused in the present embodiment is the one including no CN compound, andthe orientation film is a polyimide high-resistance orientation filmincluding no such compound as having the conjugated length of seven ormore atoms, but including the following compound.

By taking liquid crystal which are panels each manufactured byarbitrarily combining liquid crystal A to B varying in type, andorientation films A to C all satisfying such conditions, as shown inFIG. 30, a voltage holding ratio at 40 degrees is 98% or more, and theion density will be 1×10⁻¹³ MOl cm⁻³ or lower, proving that no echophenomenon occurs when moving images are displayed.

As such, according to the liquid crystal panel of the presentembodiment, the material to be used therefor has the smaller number ofimpurity ions in the liquid crystal and the orientation film, and ishigh in resistance. Therefore, no echo phenomenon occurs.

Here, even if no echo phenomenon occurs, due to slow response speed, atail phenomenon inconveniently occurs when displaying moving images.Accordingly, in view of the response speed, the guideline for propertyconstants of the liquid crystal material is described below.

The following equation (1) is an equation relating to the response speed(τ r) at a rising edge, equation (2) is an equation relating to thespeed (τ d) of a falling edge, and equation (3) is an equation relatingto the voltage of the rising edge.

$\begin{matrix}{\tau_{r} = \frac{\gamma}{{{ɛ0} \times {\Delta_{ɛ}\left( {V/L} \right)}^{2}} - {K\;{\pi^{2}/d^{2}}}}} & (1) \\{\tau_{d} = \frac{\gamma\; d^{2}}{K\;\pi^{2}}} & (2) \\{{V\; t\; h} = {\frac{\pi\; 1}{d}\sqrt{\frac{K}{{ɛ0} \times \Delta_{ɛ}}}}} & (3)\end{matrix}$

Here, in the equation (1), the equation (2), and the equation (3), γdenotes a rotational viscosity, K denotes an elastic constant, I denotesa space between electrodes, V denotes a voltage, and d denotes a gap.

The response speed is preferably 1 ms or smaller, practically theresponse speed is 40 ms or smaller since τr+τd, and preferably theresponse speed is 30 ms or smaller. As is evident from the equation (1)and the equation (2), the smaller γ will increase the response speed.The problem is that reducing γ without using the liquid crystalincluding CN group is considerably difficult under presentcircumstances. Thus, instead, try to reduce d to increase the responsespeed. In order to reduce d, there needs to be an increase Δn. If thedrive voltage between the pixel electrode and the common electrode hasany allowance, Δ∈ may be small in value. Accordingly, the guideline fordesigning the liquid crystal material is to set Δn as large as possible,and γ as small as possible within a range not affecting thecharacteristics (especially reliability) of the liquid crystal displayapparatus.

To be specific, γ preferably takes a value in the range of 100 to 140mPa·s (preferably 120 or smaller, but 130 to 140 under presentcircumstances), Δn in the range of 0.9 to 1.2 (preferably 1.1 orlarger), and Δ∈ in the range of 6 to 12 (when the drive voltage is 7.5Vor lower, preferably 9 or larger. When the drive voltage is 10V orhigher, Δ∈ may be in the range of 6 to 10).

Here, since the property constants of the liquid crystal material suchas Δn, Δ∈, and γ correlate to one another, they can be realized bymixing various types of liquid crystal, the compound and the compositionof which are not specifically limited.

In the above, various embodiments of the present invention aredescribed, and by arbitrarily combining those according to conditionsrelating to the panel structure, the material, the drive, and theperipheral circuits, the echo phenomenon can be suppressed at lowercots.

Here, the liquid crystal display apparatus is described above in thevarious embodiments, but in any display device for driving pixels withAC drive may also cause an echo phenomenon similar to the one occurringin the liquid crystal display apparatus. In this point of view, thepresent invention is not limitedly applicable only to the liquid crystaldisplay apparatus, but is widely applicable to any display devicesoperating under AC drive.

INDUSTRIAL APPLICABILITY

As described above, in accordance with the present invention, an echophenomenon can be prevented and moving image display with higher qualitycan be realized even in display devices varying in type typified byliquid crystal display apparatuses which occur the echo phenomenon whendisplaying moving images, and degrade those when displayed.

1. An image display apparatus for displaying an image based on an inputvideo signal, said image display apparatus comprising: a display devicefor outputting image lights according to a voltage to be applied; adrive unit for driving said display device by switching a drive voltageapplied across a pixel liquid crystal between positive and negative forapplication based on the input video signal; and a suppression unit forsuppressing a polarization phenomenon in said display device, whereinsaid suppression unit is operable to correct, when the input videosignal having different levels is inputted in two successive frames towhich the drive voltage having different polarities is applied, theinput video signal in at least one of the two successive frames so as tomake a difference of absolute values of the drive voltage of the twosuccessive frames to be no greater than ½ of a maximum drive voltage;and wherein the input video signal includes moving images.
 2. The imagedisplay apparatus according to claim 1, wherein when an absolute valueof a level of the input video signal in an n-th frame of a signal beingnormal to the input video signal is different from an absolute value ofthe level of the input video signal in an n+1-th frame or in an n−1-thframe, said suppression unit is operable to correct the input videosignal of any one of the n+1-th frame, the n−1-th frame, or the n-thframe.
 3. The image display apparatus according to claim 2, wherein saidsuppression unit is operable to correct the input video signals of boththe n+1-th frame and the n−1-th frame.
 4. The image display apparatusaccording to claim 2, wherein when correcting the input video signal ofat least any one of the n+1-th frame, the n−1-th frame and the n-thframe, said suppression unit is operable to hold a maximum value or aminimum value of the level of the input video signal of the n+1-thframe, the n−1-th frame, and the n-th frame.
 5. The image displayapparatus according to claim 2, wherein when correcting the input videosignal of at least any one of the n+1-th frame, the n−1-th frame, andthe n-th frame, said suppression unit is operable to hold a sum of theabsolute values or a square sum of the absolute values of the level ofthe input video signal of the n+1-th frame, the n−1-th frame, and then-th frame.
 6. The image display apparatus according to claim 1, whereinsaid suppression unit is operable to correct the input video signal ofat least one of the two successive frames to make the difference of theabsolute values of the drive voltage to be 1/10 of the maximum drivevoltage or smaller.
 7. The image display apparatus according to claim 6,wherein said suppression unit is operable to correct the input videosignal of at least one of the two successive frames when, prior tocorrection, the difference of the absolute values of the drive voltageis exceeding 1/10 of the maximum drive voltage.
 8. The image displayapparatus according to claim 1, wherein said drive unit is operable todivide one vertical scanning period of the input video signal into afirst sub period and a second sub period, output the input video signalin the first sub period, and output a comparison signal in the secondsub period.
 9. The image display apparatus according to claim 8, whereinthe second sub period is shorter than the first sub period.
 10. Theimage display apparatus according to claim 8, wherein the drive voltagein the second sub period is a voltage of a pedestal level or lower whensaid display device is a normally black type, and is a voltage of thepedestal level or higher when said display device is a normally whitetype.
 11. The image display apparatus according to claim 10, whereinsaid display device is a normally black type, and the drive voltage inthe second sub period is 0V.
 12. The image display apparatus accordingto claim 8, wherein the drive voltage in the second sub period isapplied to a plurality of scanning lines at one time.
 13. The imagedisplay apparatus according to claim 1, wherein said drive unit isoperable to scan, in an n-th frame, odd-numbered scanning lines for adata signal, and even-numbered scanning lines for a compensation signal,and scan, in an n+1-th frame, the odd-numbered scanning lines for thecompensation signal, and the even-numbered lines for the data signal.14. The image display apparatus according to claim 1, wherein during onevertical scanning period of the input video signal, after sequentiallyscanning either one of odd-numbered scanning lines and even-numberedscanning lines, said drive unit is operable to sequentially scan thescanning lines.
 15. The image display apparatus according to claim 1,wherein said drive unit is operable to apply the drive voltage of thesame polarity without polarity reverse to at least any two successiveframes.
 16. The image display apparatus according to claim 15, whereinsaid drive unit is operable to apply a drive voltage that alternates thepolarity for every two frames.
 17. The image display apparatus accordingto claim 15, wherein said drive unit is operable to apply the drivevoltage of the same polarity between any two successive frames once forevery n frames.
 18. The image display apparatus according to claim 1,wherein said display device includes: liquid crystal; and an orientationfilm; and wherein a combination of said liquid crystal and saidorientation film is a combination to make a voltage holding ratio 98% orhigher.
 19. The image display apparatus according to claim 1, whereinsaid display device includes: liquid crystal; and an orientation film;and wherein as said suppression unit, said liquid crystal includes nomore than 1% in weight of a cyano group compound, and said orientationfilm does not include a high polymer whose conjugated length is sevenatoms or more.
 20. The image display apparatus according to claim 1,wherein said display device includes: liquid crystal; an orientationfilm; and a pixel electrode and a common electrode for applying thedrive voltage to said liquid crystal; and wherein at least a part ofsaid pixel electrode and said common electrode applies the drive voltageto said liquid crystal only via said orientation film.
 21. The imagedisplay apparatus according to claim 1, wherein said display deviceincludes: liquid crystal; and an electrode for applying the voltage tosaid liquid crystal; and wherein a part of said liquid crystal is drivenin a state in which there is none of said electrode in the vicinitythereof.
 22. The image display apparatus according to claim 1, whereinsaid display device includes: liquid crystal; and a pixel electrode anda common electrode for applying the voltage to said liquid crystal; andwherein said liquid crystal is driven by an electric field which isgenerated between said pixel electrode and said common electrode, and isalmost parallel to a substrate.
 23. The image display apparatusaccording to claim 1, wherein said display device is comprised of aplurality of materials, wherein at least one of the materials is amaterial with which an echo phenomenon easily occurs.
 24. An imagedisplay method for displaying an image by driving a display device basedon an input video signal, said method comprising: driving the displaydevice by switching a drive voltage applied across a pixel liquidcrystal between positive and negative for application based on the inputvideo signal; and suppressing a polarization phenomenon in the displaydevice by correcting, when the input video signal having differentlevels is inputted in two successive frames to which the drive voltagehaving different polarities is applied, the input video signal in atleast one of the two successive frames so as to make a difference ofabsolute values of the drive voltage of the two successive frames to beno greater than ½ of a maximum drive voltage; wherein the input videosignal includes moving images.